1. Field of the Invention
The present invention relates generally to radiography and more particularly it relates to radiographic intensifying screens having improved spatial frequency characteristics (or response function) in the low spatial frequency region and being capable of providing radiographs having excellent image qualities.
2. Description of the Prior Art
Radiography is generally classified into two types, viz, medical radiography used for medical diagnosis and industrial radiography used for nondestructive inspection of industrial materials. In the both types the basic reasons for lack of definition in radiographs are the geometric unsharpness of the focal spot of the X-ray tube, the movement of the subject, the influence of radiation scattered from the subject, the response functions of intensifying screens, X-ray films, and the naked eye observing the radiographs, etc., and thus one important factor in overcoming the aforesaid faults is provision of intensifying screens with spatial frequency characteristics or response function that can improve the image qualities of radiographs.
The sharpness of intensifying screens is in a reciprocal relation to the sensitivity or screen speed of the intensifying screens and thus it is generally believed that the sensitivity of intensifying screens must be reduced to some extent for increasing the sharpness of the screens. It is also important in radiographic diagnosis to obtain radiographs having high diagnostic usefulness with as low a patient dosage as possible. In order to obtain radiographs having good image qualities without reducing the speed of intensifying screens, it is necessary to provide the intensifying screens with effective spatial frequency characteristics determined upon sufficient consideration of the whole radiographing system including the final observation of the medical radiographs.
There will now be given on analysis of a radiographing system and a description of the effective spatial frequency characteristics of intensifying screens which were induced from the results of the analysis. The detailed analysis and description which follow are presented in reference to FIGS. 1 to 4.
In analyzing the radiographing system, reference will be made to the typical radiographing conditions in the case of radiographing the stomach.
Under typical radiographing conditions, the focus-film distance is 65 mm, the focal spot size of the X-ray tube is 1.5 .times. 1.5 mm.sup.2, and the exposure time is 0.08 second. In this case the geometric unsharpness R.sub.f (.nu.) caused by the focal point of the X-ray tube expressed in terms of the response function is approximately shown by the following Gauss distribution: EQU R.sub.f (.nu.) = exp [-2.pi..sup.2 (n/3).sup.2 .nu..sup.2 (b/a).sup.2 ]
where n is the size of the focal point of the X-ray tube, .nu. is the spatial frequency, a is the focus-film distance, and b is the subject-film distance.
The distance between the stomach of a prone human body and the X-ray film is usually about 10 cm and thus upon inserting the numerical values in the above equation, the geometric unsharpness R.sub.f (.nu.) caused by the focal spot size of the X-ray tube is shown by curve a of FIG. 1.
Furthermore, the unsharpness R.sub.m (.nu.) of a radiograph caused by the movement of the stomach and expressed in terms of the response function is approximately shown by the following sampling function: EQU R.sub.m (.nu.) = (sin.pi..nu.vt/.pi..nu.vt)
where .nu. is the spatial frequency, v is the velocity of the movement of the subject, and t is the exposure time.
From cineradiographic analysis, the movement of the stomach has been generally confirmed to be about 3 mm/sec and also the average exposure time is ordinarily 0.08 second. Thus, upon inserting these numerical values, the unsharpness R.sub.m (.nu.) caused by the movement of the stomach is shown by curve b of FIG. 1. Moreover, the medium type screens are usually used in the case of radiographing the stomach and the spatial frequency characteristics of these intensifying screens are approximately shown by the following exponential function: EQU R.sub.s (.nu.) = e .sup.-.sup.A.sup..nu.
where A is a constant defined by the nature of the intensifying screen and is usually about 0.6 in the medium type screens. Thus, upon inserting this numerical value, the spatial frequency characteristics R.sub.s (.nu.) of the medium type screens are shown by curve c of FIG. 1.
From the results described above, the total sharpness R.sub.T (.nu.) in the radiographing system of the stomach can be expressed by the following equation: EQU R.sub.T (.nu.) = R.sub.f (.nu.).sup.. R.sub.m (.nu.).sup.. R.sub.s (.nu.)
The total sharpness R.sub.T (.nu.) is shown by curve d of FIG. 1.
As is clear from the results illustrated in FIG. 1, it is important in improving the image qualities in a radiographing system to increase the response function of the intensifying screen in the low spatial frequency regions (0-3.0 lines/mm). Also, as shown in FIG. 1, the total sharpness becomes nearly zero in the high spatial frequency region due to various factors even if the response function of the intensifying screen itself remains high.
Furthermore, since radiographs are observed by the naked eye for diagnosis, it is also necessary to consider the response function of the naked eye in analyzing the radiographing system. Typical examples of the response function of the naked eye reported up to this time are illustrated in FIG. 2, in which curve a is the response function of the naked eye at 43.1 radlux reported in the report entitled "Optical and Photoelectrical Analog of the Eye" by O. H. Schade; JOSA Vol. 47, 47,721 (1956) and curve b is the response function of the naked eye at 200 radlux reported in the report entitled "Response Function of Naked Eye" by Shingo Ooue; Applied Physics Vol. 28,531 (1959). Although the response function of the naked eye differs to some extent according to the observing conditions, the response function has its peak at about 0.5-1.0 line/mm and decreases rapidly with the increase in spatial frequency as shown in FIG. 2. As is clear from the result, it is effective for increasing the diagnostic usefulness of radiographs that the response function of the intensifying screen be high not in the high spatial frequency regions of 5-10 lines/mm but in the low spatial frequency regions up to 2-3 lines/mm.
An intensifying screen is essentially composed of a support and a fluorescent layer formed thereon and usually the fluorescent layer is further covered with a transparent protective layer. The fluorescent layer is composed of a fluorescent substance dispersed in a suitable resinous binder. In the preparation of intensifying screens, the thickness of the fluorescent layer, the kind of the fluorescent substance, the mixing ratio of the fluorescent substance and a resinous binder, the grain size of the fluorescent substance, etc., are properly selected upon sufficient consideration of the desired characteristics of the intensifying screen, such as sharpness, screen speed, etc. The fluorescent substance used for the fluorescent layer does not have uniform grain size but has a grain size distribution similar to the Gauss distribution. That is, it is quite difficult to prepare a fluorescent substance having uniform grain size and thus the grain size of the fluorescent substance forming the fluorescent layer is usually defined by the mean grain size and the deviation value.
The intensifying screen is usually prepared by directly applying a dispersion of a fluorescent substance in a resinous binder onto a cardboard or plastic support having a thickness of 0.3-0.5 mm maintained horizontally and after drying by heating, applying on the dried layer a transparent resinous protective composition at a thickness of about 10 microns or alternatively is prepared by applying on a smooth support base a transparent resinous protective layer about 10 microns thick followed by drying, applying thereon a dispersion of a fluorescent substance in a proper resinous binder followed by drying by heating, separating the layers from the support base, and applying the result under pressure onto an adhesive-bearing surface of a cardboard or plastic support by heating.
In both of the aforesaid production modes, the fluorescent substance grains are observed by a scanning electron microscope to be almost uniformly dispersed in a fluorescent layer 33 as shown schematically in FIG. 3 regardless of the size of the grains. Also, FIG. 4 shows a scanning electron photomicrograph of the cross section of the fluorescent layer in a conventional intensifying screen. The line image intensity distribution (the emission distribution of an intensifying screen to the linear X-ray input) of the conventional intensifying screen having the fluorescent layer as illustrated above is considerably broad as shown by curve a of FIG. 5 and as a result the intensifying screen is unsharp. Also, the conventional intensifying screen prepared by the latter manner as described above has an adhesive layer 32 between the fluorescent layer 33 and a support 31 as shown in FIG. 3 and thus in such an intensifying screen since the intensifying action thereof has been increased by rendering the support 31 light reflective, the light pass length becomes longer by twice the thickness of the adhesive layer 32 owing to the presence of said adhesive layer, and this also increases the unsharpness of the intensifying screen.